(management-ports)=

# Management ports and the SFP (OSC) port

All CzechLight devices can optionally carry an **optical supervisory channel (OSC)** at **1510 nm**.
It lets neighbouring devices communicate over the same fiber as the data traffic.
The control board used in all these devices has one SFP cage, and the front-panel label comes from this OSC use.
**The label does not mean that OSC is active.**

- **Inline devices with OSC** need an East and a West filter plus two OSC SFP modules.
  They get these through an extension that adds two more SFP ports.
- **When the product is delivered in the version without OSC.**
  It has no OSC filters and no SFP extension, only the onboard SFP port.

Without OSC, the SFP port is simply a third Ethernet port.
Fit any standard SFP module and use it like eth0 or eth1 for management, for example as a fiber management uplink.

By default the operating system **bridges all Ethernet ports together**.
In this device **eth0, eth1 and the SFP** are members of one bridge, `br0`.
The device has a single IP address, and it is on `br0` rather than on the individual ports, so you can use any of the three ports for management.
In devices with the OSC extension, the two extra SFP ports join the same bridge.

## Practical consequences

- You can daisy-chain devices: the management network enters on one port and continues out of another.
- Treat all ports as one management network: traffic arriving on one port is forwarded to the others.
- The bridge runs **MSTP (Multiple Spanning Tree Protocol), enabled by default**, which is meant to block redundant paths.